Where are the bitrate settings?
UPDATE: Starting with blender 2.79 the encoding options are available only if you enable FFmpeg Video.

A new section for Encoding is available then to choose the video container, codec and also the bitrate .

To manually input Bitrate settings the output quality must be set to

None, use constant bit-rate



For older versions of blender:

Bitrate is found under the encoding tab. Make sure you are on a video format so that the tab is visible.

The image contains some sample settings. Adjust them to fit your needs.